This invention relates to an improved phase detector and, more particularly, to a phase detector which can readily be used to detect the phase of an input signal which is received during recurrent, or periodic, intervals.
Various types of phase detecting circuits are known for producing an output signal representing the phase difference between respective input signals. Such phase detectors are used in diverse applications, such as in detecting, or demodulating, information which has been phase-modulated onto a carrier; and in servo-controlled or feedback systems wherein the phase of one signal is to be controlled in accordance with the phase of another signal.
In general, of the various types of phase detectors which can be used in such applications, it is preferable, in many instances, to use a balanced-type of phase detector circuit. Typically, such balanced-types of phase detectors include the so-called single- and double-balanced types. The single-balanced type detector which has heretofore been used offers the advantage of a relatively simple circuit configuration. However, such single-balanced type phase detector exhibits relatively low sensitivity. That is, even in the absence of an input signal, the phase detector produces an output voltage. More particularly, an output voltage e is derived as a result of various bias currents and voltages, even when no input signals are applied. However, when an input signal having zero phase difference with respect to a reference signal is present, the voltage output of this single-balanced type of phase detector is reduced to zero. If a phase difference of, for example, 90.degree. is present, the output of the phase detector may, for example, rise to the voltage e. If the phase differential of the input signal increases to 180.degree. , the phase detector output voltage may further rise to a level 2e. Since the voltage e is produced even in the absence of an input signal, it is apparent that the sensitivity of this single-balanced type of phase detector varies from e to zero and from e to 2e. Thus, if the sensitivity of the detector is considered to be from zero to 2e, the effective sensitivity of this single-balanced type of phase detector is seen to be only one-half of the full sensitivity.
As a further defect of the typical single-balanced type of phase detector circuit, if the circuit is used to detect the phase of a signal which is applied during periodic, or intermittent, time intervals, the presence of a noise signal at a time which is adjacent such time intervals will appear in the detector output. Similarly, noise which is superposed during such intermittent time intervals also will appear in the detector output.
As a still further disadvantage of typical single-balanced type of phase detectors, variations in the magnitude of the signal whose phase is to be detected causes corresponding variations in the detector output voltage. This has the result of varying the effective sensitivity of the phase detector.
Some of the problems attending the use of single-balanced type phase detectors can be overcome by using the so-called double-balanced type of phase detector. In particular, the problem of half sensitivity of the single-balanced type detector caused by the presence of an output voltage even in the absence of an input signal, is not found in the double-balanced type of detector. That is, the full sensitivity can be attained by the double-balanced type because no output voltage is provided in the absence of an input signal. However, this advantage is obtained at the cost of complex circuit configuration requiring the use of a large number of circuit elements. Hence, the double-balanced type of phase detector is seen to be a relatively expensive circuit.
Also, even though the sensitivity of the double-balanced type of detector can be improved over that of the single-balanced type, the aforenoted problem with respect to the influence of input noise on the detector output is not easily avoided. Furthermore, even in the double-balanced type of detector, variations in the effective detector sensitivity are caused by input signal amplitude changes. Thus, the output voltage of both the single- and double-balanced type of detector is increased if the input signal amplitude is high, whereas the output voltage is decreased if the input signal amplitude is low. Thus, if output voltage is to be used as a measure of the input phase differential, it is possible that fluctuations in the input signal amplitude will result in erroneous phase detections.